Methods for Preparation of Concentrated Graphene Ink Compositions and Related Composite Materials

ABSTRACT

A rapid, scalable methodology for graphene dispersion and concentration with a polymer-organic solvent medium, as can be utilized without centrifugation, to enhance graphene concentration.

This application is a divisional of and claims priority to and thebenefit of application Ser. No. 14/121,097 filed Jul. 30, 2014 andissued as U.S. Pat. No. 9,834,693 on Dec. 5, 2017, which claimedpriority to and the benefit of application Ser. No. 61/861,257 filedAug. 1, 2013 and is a continuation-in-part of and claimed priority toand the benefit of application Ser. No. 13/453,608 filed Apr. 23, 2012and issued as U.S. Pat. No. 9,079,764 on Jul. 14, 2015, which claimedpriority to and the benefit of application Ser. No. 61/478,361 filedApr. 22, 2011—each of which is incorporated herein by reference in itsentirety.

This invention was made with government support under DE-FG02-03ER15457awarded by the Department of Energy and N00014-11-1-0690 awarded by theOffice of Naval Research. The government has certain rights in theinvention.

BACKGROUND OF THE INVENTION

Graphene, a two-dimensional sp²-hybridized lattice of carbon atoms, hasgenerated intense interest due to its unique electronic, mechanical,chemical, and catalytic properties. Recent synthetic efforts havefocused on the development of high-yield and scalable methods ofgenerating graphene. These techniques include the direct exfoliation ofeither chemically modified or pristine graphene directly into varioussolvents. For example, graphene oxide (GO) can be exfoliated fromgraphite via acidic treatments. The resulting GO flakes containhydroxyl, epoxyl, carbonyl, and carboxyl groups along the basal planeand edges that render GO strongly hydrophilic. The ease of dispersing GOin solution has facilitated the preparation of GO thin films andGO-polymer nanocomposites with interesting and potentially usefulmechanical properties. However, due to the defects and consequentdisruption of the graphene band structure introduced during oxidation,GO is a poor electrical conductor. Although the level of oxygenation canbe partially reversed through additional chemical reduction steps,significant quantities of structural and chemical defects remain.Moreover, the electrical conductivity of reduced GO flakes is less thanoptimal and is certainly deficient by comparison to pristine graphene.

In an effort to circumvent such GO limitations, recent efforts havefocused on direct solution-phase exfoliation of pristine graphene.Graphene sheets can be extracted using superacids, by sonication insurfactant solutions and through use of organic solvents. For example,superacids have demonstrated an unprecedented graphene solubility of 2mg/mL through the protonation and debundling of graphitic sheets.However, the resulting solutions are not ideally suited for additionalprocessing due to their acidity-dependent solubility and highreactivity. Direct exfoliation of graphene in surfactant solutions andselect organic solvents has also been demonstrated with concentrationsup to 0.3 mg/mL and 1.2 mg/mL, respectively, but such concentrations areachieved only following prolonged sonication times—approaching 3 weeksin duration—or extended ultracentrifugation.

Concurrently, printed electronics offers an attractive alternative toconventional technologies by enabling low cost, large area, flexibledevices that have the potential to enable unique advances in variedapplications such as health diagnostics, energy storage, electronicdisplays, and food security. Among available manufacturing techniques,inkjet printing-based fabrication is a promising approach for rapiddevelopment and deployment of new material inks. The main advantages ofthis technology include digital and additive patterning, reduction inmaterial waste, and compatibility with a variety of substrates withdifferent degrees of mechanical flexibility and form-factor. Varioustechnologically important active components have been inkjet printedincluding transistors, solar cells, light-emitting diodes, and sensors.Despite these device-level advances, the ability to patternlow-resistance metallic electrodes with fine resolution remains animportant challenge, especially as the field evolves towards highlyintegrated systems.

As discussed above, graphene is a prominent contender as a metalliccomponent in printed electronic devices due to its high conductivity,chemical stability, and intrinsic flexibility. In particular, grapheneinks provide an alternative to conventional carbon-based inks that haveshown limited conductivity, especially in formulations compatible withinkjet printing. However, such an application requires the production oflarge-area graphene that can be easily manipulated into complex devicearchitectures. Some of the primary methods that are being explored forthe mass production of graphene include growth by chemical vapordeposition (CVD), sublimation of Si from SiC, and solution-phaseexfoliation of graphite or reduced graphene oxide (RGO). Among theseapproaches, solution-phase exfoliation offers significant advantagessuch as inexpensive raw materials, potential for scalability, lowthermal budget, and compatibility with additive printing techniques.Exploiting these attributes, previous studies have demonstrated inkjetprinting of RGO for organic thin-film transistor electrodes, temperaturesensors, radio frequency applications, and chemical sensors.Nevertheless, since the electrical properties of RGO are inferior tographene, inkjet printing of pristine graphene flakes is expected tohave clear advantages in electronic applications.

Graphene can be directly exfoliated by ultrasonication in selectsolvents and superacids, or through the use of additives such as planarsurfactants and stabilizing polymers, resulting in relatively small (<10μm² in area) graphene flakes. While small flakes are necessary forstable inkjet printing, they introduce an increased number offlake-to-flake junctions in percolating films, which renders them moreresistive compared to CVD grown or mechanically exfoliated graphene.Moreover, traditional solvents and surfactants employed for grapheneexfoliation leave persistent residues even following extensiveannealing, further disrupting the conductive network.

Processing complexities represent a bottleneck for fundamental studiesand end-use applications that require well-dispersed, highlyconcentrated, pristine graphene solutions. Accordingly, there remains anon-going search in the art for an improved approach to graphene solutionconcentrations—of the sort suitable for inkjet printing and relatedapplications—sufficient to better realize the benefits and advantagesavailable from graphene and related material compositions.

SUMMARY OF THE INVENTION

In light of the foregoing, it is an object of the present invention toprovide methods relating to the preparation of concentrated graphenemedia, together with corresponding compositions and composites availabletherefrom, thereby overcoming various deficiencies and shortcomings ofthe prior art, including those outlined above. It will be understood bythose skilled in the art that one or more aspects of this invention canmeet certain objectives, while one or more aspects can meet certainother objectives. Each objective may not apply equally, in all itsrespects, to every aspect of this invention. As such, the followingobjects can be viewed in the alternative, with respect to any one aspectof this invention.

It can also be an object of the present invention to provide a rapid,scalable methodology for preparation of highly-concentrated graphenemedia without impractical, time-inefficient, excessively-long sonicationand/or centrifugation procedures.

It can be an object of the present invention to provide an economical,efficient approach to the preparation of graphene solutions, dispersionsand related graphene ink compositions, using low-cost organic solvents,such compositions at concentrations sufficient, and surface tension andviscosity tunable, for a range of end-use applications.

It can also be an object of the present invention, alone or inconjunction with one or more of the preceding objectives, to provide alow temperature, environmentally benign approach to stable inkjetgraphene printing en route to the fabrication of high-conductivitypatterns suitable for flexible or foldable electronics.

Other objects, features, benefits and advantages of the presentinvention will be apparent from the summary and the followingdescriptions of certain embodiments, and will be readily apparent tothose skilled in the art having knowledge of various graphenepreparation methods and inkjet printing applications. Such objects,features, benefits and advantages will be apparent from the above astaken into conjunction with the accompanying examples, data, figures andall reasonable inferences to be drawn therefrom.

In part, the present invention can provide a method of using acellulosic polymer for preparing concentrated graphene media and relatedcompositions. Such a method can comprise exfoliating a graphene sourcematerial with a medium comprising an organic solvent at least partiallymiscible with water and a cellulosic polymer dispersing or stabilizingagent at least partially soluble in such an organic solvent; contactingat least a portion of such an exfoliated graphene medium with ahydrophobic fluid component; and hydrating such a graphene medium toconcentrate exfoliated graphene in such a hydrophobic fluid component.Without limitation, such concentration can be achieved withoutapplication of centrifugal force.

Alternatively, the present invention can provide a method of using acellulosic polymer for preparing concentrated graphene media and relatedcompositions. Such a method can comprise providing a graphene sourcematerial; exfoliating such a graphene source material with a mediumcomprising an organic solvent at least partially miscible with water anda dispersing or stabilizing agent comprising a cellulosic polymer, sucha dispersing agent at least partially soluble in such an organicsolvent; contacting at least a portion of such an exfoliated graphenemedium with an aqueous medium to concentrate exfoliated graphene in acomposition comprising graphene and such a cellulosic polymer; andcontacting such a graphene-cellulosic polymer composition with ahydrophobic fluid component. Without limitation, exfoliating a graphenesource material can be achieved through shear mixing such a materialwith such a medium.

In certain non-limiting embodiments, such an organic solvent can beselected from suitable alcohols, esters, amides, ethers, and ketones andcombinations thereof, such a solvent as can partially solubilize such acellulosic dispersing agent. In certain such embodiments, such a solventcan comprise ethanol or dimethylformamide. Regardless of solventidentity, such a dispersing/stabilizing agent can comprise a cellulosepolymer about 46- about 48% ethoxylated.

Without limitation as to identity of an organic solvent and/or adispersing agent, a hydrophobic fluid component of this invention can beselected from fluid hydrophobic components at least partially misciblewith such an organic solvent but immiscible with water. Such hydrophobiccomponents can include, without limitation, chloroform,˜C₆-˜C₈ alkanes,terpenes, terpene alcohols and combinations thereof, optionally togetherwith compositions comprising one or more such components and one or moresuitable co-dispersants. In certain embodiments, such a hydrophobicfluid component can comprise a terpineol or, alternatively, a terpineoland cyclohexanone. Regardless, such a method can utilize a graphite as agraphene source material.

Without limitation as to organic solvent, dispersing agent and/orhydrophobic fluid component, a method of this invention can compriseiterative separation of a graphene-hydrophobic fluid component from suchan organic solvent medium, and subsequent contact with another portionof such an exfoliated graphene medium. Alternatively, a method of thisinvention can, optionally, comprise iterative concentration of agraphene-cellulosic polymer composition and subsequent dispersion.Regardless, a resulting concentrated graphene ink can be deposited orprinted on a substrate component, then can be annealed to at leastpartially remove and/or decompose residual dispersing/stabilizingcellulosic agent.

In part, the present invention can also be directed to a method ofconcentrating a graphene medium. Such a method can comprise exfoliatinggraphene from a graphene source material with a medium comprising anorganic solvent selected from ethanol and dimethylformamide, and anethyl cellulose; contacting at least a portion of such an exfoliatedgraphene medium with a terpineol; adding water to the graphene medium toconcentrate exfoliated graphene within such a terpineol component;separating such a graphene-terpineol component from such a hydratedmedium; and, optionally, iterative contact of such a separatedgraphene-terpineol fluid component with additional portions of anexfoliated graphene medium, to concentrate graphene therein. Suchconcentration can be achieved absent centrifugation. A graphene inkresulting from such iterative concentrations can be applied to asuitable substrate, then annealed to remove dispersing agent.

In part, the present invention can also be directed to a method ofpreparing a graphene ink composition. Such a method can compriseexfoliating graphene from a graphene source material with a mediumcomprising an organic solvent selected from ethanol anddimethylformamide, and an ethyl cellulose; contacting at least a portionof such an exfoliated graphene medium with an aqueous (e.g., withoutlimitation, an aqueous NaCl solution) medium to concentrate exfoliatedgraphene and ethyl cellulose; and contacting such a graphene-cellulosecomposition with a hydrophobic fluid (e.g., without limitation,comprising a terpineol and cyclohexanone) component. In certainnon-limiting embodiments, exfoliating a graphene source material cancomprise or can be achieved by shear mixing such a material and such amedium. Regardless, a resulting graphene ink composition can be appliedto, deposited and/or printed on a suitable substrate then annealed.

Accordingly, the present invention can also be directed to a compositioncomprising graphene, a hydrophobic fluid component and a graphenedispersing/stabilizing agent at least partially soluble in such ahydrophobic fluid component. Without limitation, such adispersing/stabilizing agent can comprise ethyl cellulose. In variousembodiments, regardless of dispersing/stabilizing agent, such ahydrophobic fluid component can comprise a component selected fromterpenes, terpene alcohols and related compositions. In certain suchembodiments, such a hydrophobic fluid component can comprise a mixtureof terpineol and cyclohexanone. Thermal annealing can provide such acomposition comprising a decomposition product of ethyl cellulose.

Regardless, an ink composition of this invention can comprise a grapheneconcentration of up to or greater than about 3 mg/ml. Without limitationas to any particular graphene concentration, such a composition cancomprise small, unagglomerated graphene flakes, such a morphology as canbe evidenced by atomic force microscopy. Regardless, in certainembodiments, such a composition can be printed or patterned on asubstrate and annealed, providing such a printed composition aconductivity of greater than about 2×10⁴ S/m.

The present invention can, in part, be directed to a compositecomprising such a graphene ink composition coupled to a flexible orfoldable polymeric substrate component, such a graphene composition ascan be inkjet printed on such a substrate. Such a composition can beconsidered as comprising an annealation/decomposition product of ethylcellulose-stabilized graphene. Regardless, with respect to such an inkcomposition, print morphology, electrical performance and mechanicalproperties can be substantially maintained over repeated substratebending or folding.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the Office upon request and paymentof the necessary fee.

FIGS. 1A-B. (A) Digital images of vials of a 1:5 mixture of terpineoland ethyl cellulose stabilized graphene-ethanol solution before andafter water addition. As shown by the images, upon the addition ofwater, the hydrophobic graphene flakes preferentially separate into theconcentrated terpineol fraction, leaving behind an ethanol and watermixture. (B) The concentration factor of graphene (C₀=102.4 μg/mL) isplotted after each solvent exchange concentration and graphene-ethanoladdition step for three iterations.

FIG. 1C. UV-vis absorbance spectra for graphene dispersed in DMF (upperplot) and 1% w/v EC-DMF (lower plot). Due to the high grapheneconcentration of the EC-DMF dispersion, the sample was diluted 4× in DMFto obtain a clear absorbance spectra.

FIGS. 2A-D. (A) Histograms of flake thickness for the initiallyexfoliated and third-iteration concentrated graphene solutions. (B)Digital scanning electron micrograph (SEM) images of a graphene-ethylcellulose nanocomposite fracture surface. (C) Optical transmittanceversus sheet resistance for annealed transparent conductive thin filmsblade coated from the concentrated graphene inks. (D) Digital SEM imageof an annealed graphene thin film.

FIGS. 3A-B. (A) Digital AFM image of graphene flakes deposited on SiO₂.(B) Line scan profiles of two deposited graphene flakes, with the largerflake exhibiting edge folding.

FIG. 4. Optical transmittance spectra for the five graphene conductivefilms analyzed.

FIG. 5. Representative Raman spectra of the annealed graphene thin filmand graphene-EC nanocomposite. These spectra were obtained by combiningfive individual spectra from different locations of each film and withthe intensity of the highest peak normalized to unity.

FIG. 6. Digital SEM image of an EC film fracture surface withoutgraphene. The absence of the fracture terraces, in contrast to thoseobserved in FIG. 2B, indicates that the anisotropic fracture behavior ofthe EC-graphene nanocomposite results from aligned graphene flakes.

FIGS. 7A-B. (A) absorbance spectra for dispersions of single-walledcarbon nanotubes, showing enhanced debundling and concentration usingethyl cellulose-ethanol (upper plot), in accordance with this invention.(The reference dispersion also illustrates the utility ofmethylpyrrolidone as an organic solvent component, in accordance withthis invention.) (B) a digital SEM image of an annealed SWCNT thin film.

FIGS. 8A-F. Schematic illustration of the ink preparation method. (A)Graphene is exfoliated from graphite powder in ethanol/EC by probeultrasonication. A graphene/EC powder is then isolated following (B)centrifugation-based sedimentation to remove residual large graphiteflakes and (C) salt-induced flocculation of graphene/EC. (D) An ink forinkjet printing is prepared by dispersion of the graphene/EC powder in85:15 cyclohexanone:terpineol. Digital images of (E) vial of theprepared graphene ink and (F) drop formation sequence for inkjetprinting, with spherical drops forming after˜300 μm.

FIGS. 9A-C. Characterization of graphene flakes. (A) A representativedigital AFM scan of the graphene flakes that was used to obtain particlestatistics. Histograms of (B) flake thickness and (C) flake area for 355and 216 flakes, respectively.

FIGS. 10A-B. TGA of pure EC (black) and graphene/EC composite powder(red), showing (A) mass as a function of temperature and (B) thedifferential mass loss. For the composite powder, the decompositionpeaks of EC in (B) are shifted to different temperatures due to thepresence of graphene.

FIG. 11. Shear viscosity of the graphene ink over a shear rate range of10-1000 s¹ at temperatures of 25, 30, 35 and 40° C.

FIG. 12A-D. Morphology of inkjet printed graphene features onHMDS-treated Si/SiO₂. Digital scanning electron micrographs of (A)multiple printed lines and (B) a single printed line and drop (inset,scale bar corresponds to 40 μm) illustrate the uniformity of the printedfeatures. (C) A digital atomic force microscopy (AFM) image of a singleline following 10 printing passes that shows no coffee ring features.(D) Averaged cross-sectional profiles of printed lines after 1, 3, and10 printing passes, which demonstrate the reliable increase in thicknessobtained after multiple printing passes. The cross-sectional profilesare obtained from the averaged AFM height profile over˜20 μm asindicated by the boxed region in (C).

FIGS. 13A-D. Electrical characterization of graphene features. (A)Electrical resistivity of blade-coated films plotted against annealingtemperature for a fixed annealing time of 30 minutes, showing effectivebinder decomposition at 250° C. and increased resistivity due tographene oxidation above 400° C. (B) Dependence of electricalresistivity on annealing time for a fixed annealing temperature of 250°C., showing that low resistivity is achieved following annealing for 20minutes. (C) Thickness of inkjet printed graphene lines on HMDS-treatedSi/SiO₂ for increasing numbers of printing passes. (D) Electricalresistivity of the printed features for increasing numbers of printingpasses, showing relatively stable performance after only 3 printingpasses.

FIGS. 14A-D. Digital SEM images of printed lines annealed to (A, C) 250°C. and (B, D) 450° C. (C) and (D) are higher magnification SEM images ofthe highlighted area (yellow box) from images (A) and (B), respectively.Following 450° C. annealing, the EC residue is removed, leading to asparse graphene network. This observation suggests the importance of ECdecomposition products in maintaining electrical and mechanicalintegrity of the printed features.

FIGS. 15A-E. Flexibility assessment of printed graphene lines on Kapton®substrates. (A) Resistance of graphene lines folded to a radius ofcurvature of 0.9 mm (blue, bending strain: 6.9%) and 3.4 mm (red,bending strain: 1.8%) normalized to the resistance prior to bending. (B)Normalized resistance of graphene lines measured in a flexed state forvarious degrees of bending, showing reliable retention of electricalconductivity across all measured flex states. (C) Normalized resistanceof graphene lines while measured in a folded state, showing a small andirreversible increase in resistance following folding. Digital images ofthe sample in the (D) original and (E) folded state.

FIGS. 16A-D. Shear mixing of graphene. (A) Graphene concentration forshear mixing as a function of time and centrifuge rate, highlightingdata points for sonication and shear mixing batch processes. (B)Representative digital AFM image of graphene flakes produced by shearmixing. (C) Flake thickness and (D) flake area distributions forgraphene produced by shear mixing.

FIG. 17. Photonic annealing of graphene patterns. Sheet resistance ofgraphene patterns with different post-processing conditions, includingthermal annealing, photonic annealing, and combined thermal and photonicannealing. (inset) Digital optical image of conductive graphene patternson polyethylene terephthalate (PET) following combined thermal andphotonic annealing.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS

Without limitation, various embodiments of this invention demonstrate analternative strategy for enhancing graphene exfoliation using apolymer-organic solvent composition. More specifically, as relates tocertain such embodiments, a room-temperature, ultracentrifuge-freeconcentration technique can be used to generate graphene concentrationsin excess of 1 mg/mL in organic solvents that otherwise yield poorgraphene dispersability. The resulting graphene inks are amenable tofurther processing, including casting into aligned graphene-polymernanocomposites and blade coating to form thin films, as a result oftheir low solvent boiling point and non-causticity. Because the presentinvention avoids oxidative conditions, the graphene maintainssuperlative electronic properties, which can be exploited inapplications that require highly conductive, mechanically flexible, andsolution-processable coatings.

Due to the large mismatch between the surface energies of ethanol andgraphite, ethanol is a relatively poor solvent for graphene exfoliation,yielding a post-sedimentation concentration of 1.6 μg/mL. (See,Hernandez, Y.; Lotya, M.; Rickard, D.; Bergin, S. D.; Coleman, J. N.;Langmuir 2010, 26, 3208-3213.) To overcome this limitation, a cellulosicpolymer was used to enhance the ability of ethanol to exfoliate andsuspend graphene flakes. Such polymers include, but are not limited toethyl cellulose, methyl cellulose, hydroxyethyl cellulose, carboxymethylcellulose, and hydroxypropylmethyl cellulose. Using ethyl cellulose(EC), a solution of 50 mg/mL natural graphite flakes in 1% w/vEC-ethanol was sonicated for 3 hr and centrifuged at 7,500 rpm for 4.5hr to remove the fast sedimenting graphite flakes. The resultingsupernatant provides primarily few-layer graphene sheets. Opticalabsorbance was taken to determine the graphene concentration using anabsorption coefficient of 2,460 L/g·m at 660 nm. Without limitation asto any one theory or mode of operation, addition of up to about 1% ormore EC significantly enhanced the graphene exfoliation efficiency byproviding steric stabilization of the exfoliated flakes, yielding apost-sedimentation concentration of 122.2 μg/mL. Despite thisimprovement, still higher concentrations were desired to generategraphene inks that can be easily deposited and patterned.

Towards this end, an iterative solvent exchange was employed as a rapidroom-temperature process to concentrate graphene solutions—without theapplication of centrifugal force. Various hydrophobic fluid solventcomponents at least partially miscible with an organic solvent such anethanol (or e.g., dimethylformamide or methylpyrrolidone), but notmiscible with an aqueous solvent component (e.g., ethanol and water) canbe utilized. In particular, a 1:5 volume ratio solution of terpineol andsedimented graphene solution was prepared and mixed to yield a solutionwith an initial graphene concentration of C₀=102.4 μg/mL. Water, fourtimes the volume of this initial solution, was then added to form ahydrophilic ethanol solution. Again, without limitation to theory ormode of operation, because of the hydrophobicity of the EC-stabilizedflakes, graphene is believed preferentially concentrated into theterpineol band on top of the ethanol-water solution (FIG. 1A). Thisterpineol phase was then harvested and additional sedimented graphenesolution was added for the next concentration iteration. Concentrationfactors, C/C₀, were determined after each step through opticalabsorbance for three concentration iterations (FIG. 1B). As expected,the concentration factors correspond roughly to the volumetric reductionof the graphene solution, producing a highly concentrated graphene inkat 1.02 mg/mL after three iterations. Additional iterations of solventexchange yielded diminishing returns as the viscosity of the grapheneink begin to interfere with material separation within the system. Inorder to verify the absence of flake agglomeration during theconcentration process, atomic force microscopy was performed on over 140flakes deposited from the sedimented graphene solution and the thirditeration graphene ink. Both media exhibited similar flake thicknesshistograms peaked at approximately 1.6-1.8 nm (FIG. 2A), suggestingminimal graphene agglomeration during the concentration process.

Graphene-polymer nanocomposites were solution cast from these grapheneinks. The height reduction associated with anisotropic volumecontraction during solvent evaporation resulted in the directionalalignment of the graphene flakes within the nanocomposite. In FIG. 2B,this alignment is evident on the fracture surface in the form of shearedterraces orthogonal to the direction of the volumetric contraction. Thelack of protruding graphene flakes on the fracture surface is not onlyindicative of flake alignment but also suggests strong interactionsbetween the polymer and graphene.

The electrical properties of thin films derived from the concentratedgraphene ink were assessed via transparent conductor measurements. Dueto their enhanced rheology, film forming capability, and dispersionstability, EC-stabilized graphene inks are amenable to blade coatingonto a broad range of substrates. For example, graphene inks were bladecoated onto glass slides at varying thicknesses, annealed at 400° C. for30 min in air, and rinsed with acetone to produce transparent conductivethin films. Four point probe measurements of the film sheet resistanceindicate that their electrical performance compare favorably to filmsdeposited by vacuum filtration from sedimented surfactant graphenesolutions (FIG. 2C). Electron microscopy performed on these conductivegraphene thin films (FIG. 2D) reveals a disordered network of grapheneflakes with lateral dimensions ranging from approximately 50-400 nm.Raman spectra provide further evidence that these graphene thin filmspossess low defect densities and negligible oxidation.

As demonstrated, efficient graphene exfoliation can be achieved inethanol through polymeric stabilization using ethyl cellulose. Theresulting graphene solutions can be concentrated via rapid,room-temperature, ultracentrifugation-free iterative solvent exchange,ultimately yielding stable graphene inks at mg/mL levels. Theoutstanding processability and electrical properties of the resultinginks enable the straightforward production of functional graphene-basedmaterials including highly anisotropic polymer nanocomposites andtransparent conductive thin films. Such results can promote ongoingefforts to understand and exploit solution-processable pristine graphenefor fundamental studies and device applications.

Relating to certain such embodiments of this invention, graphene inkswere produced by the exfoliation of graphite in ethanol and ethylcellulose (EC), as described more fully below. Generally, such a processprimarily produces few-layer graphene sheets, with typical thicknessesof˜2 nm and areas of˜50×50 nm² (FIG. 9). Processing steps areillustrated schematically in FIG. 8. In particular, excess graphite andEC were used to achieve high yields of suspended graphene (>0.1 mg/mL).Sedimentation-based centrifugation was then employed (FIG. 8A-B) toremove remaining large graphite flakes, yielding a dispersion of˜1:100graphene/EC in ethanol. To remove excess EC and solvent, aroom-temperature method based on the flocculation of graphene/EC wasdeveloped. Specifically, upon the addition of NaCl(aq), a solidcontaining graphene and EC was flocculated and collected following ashort centrifugation step (FIG. 8C). This graphene/EC solid wassubsequently washed with water and dried, yielding a black powder with agraphene content of˜15% (FIG. 10), which is significantly higher thanthe graphene/EC ratio in the original dispersion. Because ECencapsulates graphene flakes in solution, no irreversible aggregation ofgraphene was observed. The resulting powder is readily dispersed in avariety of solvents, allowing for the tailoring of inks for a range ofdeposition methods. In particular, dispersion of this material in selectorganic solvents (FIG. 1D-E) enables deposition of graphene by inkjetprinting (FIG. 8F).

Inkjet printing requires careful tailoring of the viscosity and surfacetension of the ink formulation to achieve stable droplet formation. Thewetting and drying properties of the ink must also be tuned to achieveproper morphology of the printed features. Furthermore, inks should notpossess large particles or volatile solvents since these components canlead to clogging of the inkjet printhead. Finally, a high concentrationof graphene is desired to reduce the number of necessary printingpasses. To achieve these goals, the graphene/EC powder was dispersed ina 85:15 mixture of cyclohexanone:terpineol (FIG. 8D). At a concentrationof 2.4 wt % graphene/EC composite (˜3.4 mg/mL graphene), this ink has asurface tension of˜33 mN/m and a high shear rate (100-1000 s⁻¹)viscosity of 10-12 mPa·s at 30° C. (FIG. 11).

The relatively low surface tension of this ink is designed for properwetting of low surface energy substrates applicable to flexibleelectronics. To assess the electrical characteristics of the ink, awell-defined substrate of Si/SiO₂ with 300 nm thermally grown oxide wasused. For a more suitable model of wetting and drying on low surfaceenergy substrates, the Si/SiO₂ substrate was treated withhexamethyldisilazane (HMDS) to decrease the surface energy. Printing wascarried out at 30° C. using a Fujifilm Dimatix Materials Printer (DMP2800) with a cartridge designed for a 10 pL nominal drop volume. Dropspacing for all printed features was maintained at 20 μm. Stableprinting of graphene lines on HMDS-treated Si/SiO₂ yielded a line widthof˜60 μm, as shown in FIG. 12A-C. The highly uniform dome-shapedcross-sectional profile across the lines provides evidence forsuccessful ink formulation, specifically showing no undesirable coffeering effects. Importantly, this advantageous cross-sectional profile wasmaintained after multiple printing passes, as shown in FIG. 12D. Thisexcellent morphology of the printed features is, without limitation,believed attributable to the suppression of the coffee ring effectthrough a Marangoni flow established by the surface tension gradientthat develops due to solvent evaporation. This flow homogenizes thedroplet composition, resulting in a uniform morphology of the printedfeatures. Again, without limitation to any one theory or mode ofoperation, the sp²-bonding and small lateral size of the graphene flakesminimizes folding or buckling of the printed flakes, which promotes lowsurface roughness and well-defined flake-flake contacts.

The polymeric binder EC encapsulates graphene flakes following solventevaporation, and subsequent thermal annealing can be employed to obtainhighly conductive features. To study the electrical behavior of thecomposite material as a function of annealing conditions, films wereblade-coated on glass slides and annealed in an ambient atmosphere withsystematic variations in the annealing time and temperature. As shown inFIG. 13A, a 250-350° C. anneal for 30 minutes results in highconductivity graphene films. At 250° C., annealing for as short as 20minutes was sufficient to achieve low resistivity (FIG. 13B). For theremainder of this study, the annealing temperature and time of 250° C.and 30 minutes, respectively, were chosen to enable compatibility withflexible electronics applications.

For a detailed assessment of the electrical performance of the printedfeatures, 4 mm long lines with varying thicknesses were printed onHMDS-treated SiO₂ and annealed at 250° C. for 30 minutes. The linethickness increases linearly with the number of printed layers, witheach layer adding˜14 nm to the thickness (FIG. 13D). The lineresistivity reaches a relatively stable low value after only 3 printingpasses, owing to the high concentration of the ink and the excellentmorphology of the printed features (FIG. 13C). The measured conductivityof 2.5×10⁴±0.2×10⁴ S/m (resistivity of 4×10⁻³±0.4×10⁻³Ω cm) for theprinted lines after 10 printing passes is˜250 times higher thanpreviously reported for inkjet printed graphene. This dramaticimprovement indicates the effectiveness of the method presented here,which avoids the graphene degradation that occurs in competing processesbased on ultrasonication of graphene in harsh solvents.

Thermal gravimetric analysis (TGA) of the ink indicates that ECdecomposition occurs in two stages, with a low temperature charringbeginning below 250° C. and volatilization and removal of the EC residueoccurring at temperatures above 400° C. (FIG. 10). This observationcoupled with the high electrical conductivity observed after annealingat temperatures of 250-350° C. suggests that the initial decompositionof EC enables efficient charge transport through the graphene network.Because cellulose derivatives can thermally decompose into aromaticspecies, any resulting pi-pi stacking between the residues and thegraphene flakes provides relatively efficient charge transport. Inaddition, the increase in resistivity upon annealing at 400-450° C.correlates well with the removal of residue from the film in the secondstage of EC decomposition. Furthermore, the EC residue creates a denseand continuous film, as determined from scanning electron micrographs ofprinted lines following annealing at 250° C. and 450° C. (FIG. 14). Thisfilm densification could potentially enhance the mechanical propertiesof the printed graphene features and enable a robust tolerance forbending stresses in flexible applications.

To assess mechanical properties, lines were printed on polyimide (DuPontKapton® 125 μm) substrates and annealed at 250° C. for 30 minutes. (Sucha polyimide is representative of a range of flexible polymeric materialsof the sort well-known to those skilled in the art and available for useas a bendable/foldable substrate.) Various flexibility tests wereemployed to characterize these printed graphene lines. For example, toinvestigate the reliability over a large number of bending cycles, theelectrical resistance was measured up to 1000 cycles. As shown in FIG.15A, there is no observable degradation in the line conductivity for abending radius of curvature of 3.4 mm. Even at a radius of 0.9 mm, theresistance remained nearly unchanged after a marginal initial increase.At this radius of curvature, some cracking was observed in thesubstrate, which suggests that the small loss of conductivity is alimitation of the substrate rather than the printed features. Theelectrical performance of the printed features was also measured underapplied stress for various radii of bending (FIG. 15B), with no observedloss in conductivity. As a final test, the resistance of the graphenelines was measured in a folded state, as shown in FIG. 15C-E, againresulting in only a slight decrease in conductivity on the order of 5%that can again be likely attributed to substrate cracking. Overall,these mechanical tests show the utility of the present graphene inks inflexible, and possibly even foldable, electronic applications.

As shown by the preceding, this invention provides a graphene ink from agraphene/EC powder produced using only room temperature processingmethods. The graphene/EC powder allows for careful tuning of the ink toachieve stable inkjet printing of features on a variety of substrateswith excellent morphology, and can be applied to other printingtechniques in a straightforward manner. In addition, the conductivity ofprinted features following mild annealing is over two orders ofmagnitude better than previously reported for inkjet printed graphenedespite a smaller flake size, indicating efficient flake-flake chargetransport. Such results are believed enabled by a synergistic EC binderfor graphene exfoliation, which reduces flake-flake junction resistanceupon annealing relative to graphene films containing residual solvent orsurfactant. Finally, low processing temperatures enable compatibilitywith flexible substrates, thereby allowing demonstration of the hightolerance of printed graphene features to bending stresses. With thisunique combination of attributes, the graphene-based inks of thisinvention can find utility in a wide range of printed, flexible, and/orfoldable electronic applications.

EXAMPLE OF THE INVENTION

The following non-limiting examples and data illustrate various aspectsand features relating to the methods and/or compositions of the presentinvention, including the preparation and use of concentrated graphenesolutions, graphene ink compositions and related composites, as aredescribed herein. In comparison with the prior art, the present methodsprovide results and data which are surprising, unexpected and contrarythereto. While the utility of this invention is illustrated through theuse of several graphene dispersion agents and hydrophilic organicsolvents, together with several hydrophobic fluid components which canbe used therewith, it will be understood by those skilled in the artthat comparable results are obtainable with various other dispersionagents and hydrophilic or hydrophobic solvents, as are commensurate withthe scope of this invention.

Example 1a

Exfoliation and Sedimentation Processing of Graphene. 2.5 g of naturalgraphite flake (3061 grade, Asbury Graphite Mills) was added to 50 mL of1% w/v ethyl cellulose (EC) (Aldrich) ethanol (EtOH) solution inside aplastic 50 mL centrifuge tube (note that Aldrich does not explicitlyprovide the molecular weight of its EC; rather, the viscosity isspecified (e.g., 4 cP) when the EC is loaded at 5% w/v in 80:20toluene:ethanol). Two tubes containing this mixture were simultaneouslysonicated in a Bransonic 3510 tabletop ultrasonic cleaner for 3 hr at 40kHz and 100 W. In order to efficiently sediment out the graphite flakes,the centrifugation was performed in a two-step process. First, thesonicated graphene dispersions were centrifuged in a large volumecentrifuge (Beckman Coulter Avanti J-26 XP Centrifuge) for 10 min at7,500 rpm to remove the fast sedimenting graphite flakes. Thesupernatant was then decanted from each 50 mL centrifuge tube andcombined. A second sedimentation step was then performed on thiscombined solution in two 250 mL tubes for 4.5 hr at 7,500 rpm or anaverage relative centrifugal force (RCF) of 6,804 g.

Example 1b

Thermal Stability of Polymer Enhanced Graphene Dispersions. Experimentswere undertaken to highlight the thermal stability of EC-based graphenedispersions, of the sort discussed above, especially in comparison totraditional surfactant-based dispersions. Here, graphene dispersions in1% w/v EC-EtOH and 1% w/v sodium cholate-water (SC-H₂O, prior art) wereproduced using the sonication and centrifugation procedures detailedabove. Both dispersions were then concentrated to˜1 mg/mL via thermalevaporation.

At elevated temperatures, graphene flakes in the SC-based dispersionagglomerate rapidly to form precipitates, while the EC-based dispersionremains well dispersed. To quantify their thermal stabilities, bothconcentrated dispersions were diluted to 0.1 mg/mL and centrifuged at15,000 rpm for 1 min. The UV-vis absorbance spectra for theirsupernatants were then obtained. Using the same absorbance coefficientdiscussed above (2460 L/gm at 660 nm), it was determined that 97.7% ofthe graphene remained suspended in the EC-EtOH medium, while only 18.1%remained suspended in the SC-H₂O solution. The stability of thesepolymer-based graphene dispersions can be exploited in subsequentpost-synthetic processing.

Example 1c

Enhanced Graphene Production Efficiency in DMF. Improvement in grapheneproduction is also demonstrated by adding EC to dimethylformamide (DMF),which has moderate intrinsic graphene solubility. In this case, naturalgraphite was bath sonicated for 3 h at 50 mg/mL in both DMF and 1% w/vEC-DMF. After centrifugation at 7500 rpm for 4.5 h to remove the thickgraphite flakes, UV-vis absorbance spectra were taken to assess theirgraphene concentrations (FIG. 1C).

Using an absorbance coefficient of 2460 L/g·m at 660 nm, the grapheneconcentration for the DMF and EC-DMF dispersions were determined to be14.1 and 82.8 μg/mL, respectively. (See, Hernandez, Y.; Nicolosi, V.;Lotya, M.; Blighe, F. M.; Sun, Z.; De, S.; McGovern, I. T.; Holland, B.;Byrne, M.; Gun'Ko, Y. K.; Boland, J. J.; Niraj, P.; Duesberg, G.;Krishnamurthy, S.; Goodhue, R.; Hutchison, J.; Scardaci, V.; Ferrari, A.C.; Coleman, J. N., Nat. Nanotechnol. 2008, 3, 563-568). It followsthat, the addition of 1% w/v EC to DMF yielded a 5.9-fold improvement inthe graphene exfoliation/production efficiency. Overall, improving thegraphene exfoliation efficiency in organic solvents with moderate tohigh intrinsic graphene solubilities can both reduce material waste andbenefit printed electronic and related applications where highergraphene-to-dispersant ratios are required.

In accordance with this invention, without limitation, various otherC₂-C₅ alcohols, esters, ethers, ketones and amides can be used, inconjunction with a cellulosic polymer, to suspend and exfoliategraphene.

Example 2

Graphene Concentration via Iterative Solvent Exchange. To ensure properhydrophobic phase separation, water, in excess of four times the volumeof the starting graphene solution, is added. A brief sonication step, ofapproximately 1 min, is also performed after each graphene concentrationand graphene addition step to facilitate phase separation and solutionmixing.

Example 3

SiO₂ Graphene Deposition. Graphene flakes from both the sedimentedgraphene solution and third-iteration concentrated graphene solutionwere deposited onto 100 nm thick oxide silicon wafers for imaging. Thewafers were first submerged in 2.5 mM 3-aminopropyl triethoxysilaneaqueous solution to functionalize the surface with a hydrophobicself-assembling monolayer for 30 min. The substrates were then rinsedwith water and dried under a stream of N₂. Both graphene solutions werethen diluted to approximately 0.02 mg/mL in ethanol after which a dropof each was placed onto the functionalized wafers for 10 min. The dropswere then blown off under a stream of N₂, and the wafer was rinsed withwater. To remove the residual EC, the wafers were annealed for 20 min at400° C. in air.

Example 4

Atomic Force Microscopy Thickness Measurements. All atomic forcemicroscopy (AFM) images were obtained using a Thermo MicroscopesAutoprobe CP-Research AFM in tapping mode using cantilever B onMikroMasch NSC NSC36/Cr-AuBS probes. 2 μm ×2 μm images were collectedusing identical scanning parameters. Flake thicknesses were determinedusing line-scan thickness profiles across flakes larger than 5,000 nm²while avoiding regions where EC residues were present. (FIG. 3) 146flakes were analyzed on the wafer deposited with the sedimented graphenesolution, and 156 flakes were analyzed for the wafer deposited with thethird-iteration concentrated graphene solution.

Example 5

Thin Film Deposition. Graphene-EC and graphene thin films were bladecoated from concentrated graphene inks onto glass slides using either 1or 2 layers of 3M Scotch Magic Tape (about 30-about 40 μm per layer) asmasks. In order to optimize ink rheology for uniform film deposition,10% w/v EC (Aldrich, 22 cP, 5% in toluene:ethanol 80:20) in ethanol wasadded to the graphene ink. The modified graphene ink was deposited into2 cm ×2 cm squares on 2.54 cm ×2.54 cm silica glass slides. To obtainfilms with different optical densities, select films were also spun at10,000 rpm for 3 min. These films were then allowed to dry overnight,and the mask was removed to obtain a transparent graphene-polymer film(not shown). Graphene thin films require an additional annealing step,performed for 30 min at 400° C. in air, to remove the EC and enhanceflake-to-flake contact. After annealing, these graphene thin films wererinsed in acetone before optical transmittance and four point probemeasurements.

Example 6

Optical Absorbance and Transmittance Measurement. Optical absorbancemeasurements to determine graphene solution concentrations andtransmittance measurements for transparent conductive graphene thinfilms were performed using a Varian Cary 5000 spectrophotometer.Background from the optical cuvette, EC-ethanol solution, and glassslide were subtracted from the spectra of the graphene dispersions andfilms. Due to their high absorbance, concentrated graphene solutionswere diluted either 4× or 10× to ensure that the optical absorbance waswithin the detector limits. As expected, the graphene thin films of thepreceding example provide featureless optical absorbance spectra withhigh transparency at visible and infrared wavelengths (FIG. 4).

Example 7

Raman Spectroscopy of the Graphene Films. Raman spectroscopy wasobtained using a Renishaw inVia Raman microscope with an excitationwavelength of 514 nm. Five spectra were obtained on different areas ofthe annealed graphene film and the graphene-EC nanocomposite using abeam size of 1-2 μm, allowing multiple flakes to be probed in eachmeasurement. These spectra showed minimal variation across differentlocations and were combined to form a representative Raman spectrum forthe entire film (FIG. 5). Typical Raman spectra for the annealedgraphene film exhibit four primary peaks: the G band at˜1,590 cm⁻¹, 2Dband at˜2,700 cm⁻¹, and the disorder-associated D and D′ bands at˜1,350cm⁻¹ and˜1,620 cm⁻¹ respectively. The intensity ratio of the D and Gbands, I(D)/I(G), is a measure of the level of defects that areintroduced during the sonication and annealing processes. The I(D)/I(G)value for the annealed graphene film was˜0.38, significantly less thanreported values for surfactant exfoliated graphene solutions with asimilar size distribution (˜0.93) (see, Green, A. A.; Hersam, M. C.;Nano Lett. 2009, 9, 4031-4036) and heavily reduced graphene oxide(˜0.82) (Gao, W.; Alemany, L. B.; Ci, L.; Ajayan, P. M. Nature Chem.2009, 1 (5), 403-408) but higher than that for larger-sized solventexfoliated graphene flakes. (See, Hernandez, Y.; Nicolosi, V.; Lotya,M.; Blighe, F. M.; Sun, Z.; De, S.; McGovern, I. T.; Holland, B.; Byrne,M.; Gun'Ko, Y. K.; Boland, J. J.; Niraj, P.; Duesberg, G.;Krishnamurthy, S.; Goodhue, R.; Hutchison, J.; Scardaci, V.; Ferrari, A.C.; Coleman, J. N. Nat. Nanotechnol. 2008, 3, 563-568.) The measuredvalue of˜0.38 indicates that large quantities of defects or oxidationwere not introduced during exfoliation and annealing.

Example 8

Nanocomposite Fracture Surface. The graphene-EC and graphene-free ECfilms were fractured using shearing forces applied orthogonally to theplanes of the films. The fractured surfaces were then analyzed using SEMto gauge the adhesion strength of graphene to EC and orientation ofgraphene flakes. (See, FIG. 6.)

Example 9

Scanning Electron Microscopy. Scanning electron microscopy of thetransparent conductive graphene thin films and fracture surfaces ofgraphene-EC nanocomposites was performed on a Hitachi 4800 scanningelectron microscope using a 1 kV accelerating voltage.

Example 10

Dispersion and iterative solvent exchange can be used concentrate fluidmedia comprising other nanodimensioned materials, such as single-walledcarbon nanotubes, using procedures analogous to those described inexamples 1-2. For instance, single walled carbon nanotubes (SWCNTs) weredispersed in 1% EC-EtOH via lh horn sonication and 4.5 h centrifugationat 7500 rpm. Compared to a reference 0.04 mg/mLSWCNT/N-methylpyrrolidone (NMP) dispersion, without EC, theconcentration of the 1% EC-EtOH dispersion was determined to be around0.75 mg/mL (see, FIG. 7A). Solvent exchange with terpineol provided aconcentrated SWCNT-EC ink. Likewise, substrate deposition and materialcharacterization can be accomplished, using techniques of sort describedin examples 3-9. A transparent SWCNT thin film was prepared by bladecoating and annealing the aforementioned ink at 400° C. in air for 30minutes. An SEM image of the annealed SWCNT thin film is shown in FIG.7B.

Example 11

Solvent Exfoliation and Processing of Graphene. 10.0 g natural graphiteflake (Asbury Graphite Mills, 3061 Grade) was dispersed in a solution of200 mL, 2% w/v ethyl cellulose (EC) in ethanol (EC: Aldrich, viscosity 4cP, 5% in toluene/ethanol 80:20, 48% ethoxy; ethanol: Koptec, 200 proof)in a stainless steel beaker. The dispersion was sonicated using a probesonication system (Fisher Scientific Sonic Dismembrator Model 500, 13 mmBranson tip) for 90 minutes at 50 W in an ice water bath. The resultingdispersion was centrifuged (Beckman Coulter Avanti® J-26 XPI) at 7,500rpm (10,000 g) for 15 minutes, and the supernatant was collected. Tothis dispersion, a 0.04 g/mL aqueous solution of NaCl(Sigma-Aldrich, >99.5%) was added in a 1:2 volume ratio. The resultingmixture was centrifuged at 7,500 rpm for 8 minutes, after which thesupernatant was removed. The resulting graphene/EC solid was dried,dispersed in ethanol, and passed through a 5 μm sieve (IndustrialNetting, BS0005-3X1) to remove any large particles that might compromiseinkjet printing. The dispersion was then flocculated again, with thesame parameters as above. To remove any residual salt, the resultinggraphene/EC solid was washed with deionized water and isolated by vacuumfiltration (Millipore Nitrocellulose HAWP 0.45 μm filter paper). Thisisolated graphene/EC product was then dried, yielding a fine blackpowder. (Related graphene exfoliation and concentration procedures, withalternate ordering of steps and/or techniques, are as described in theaforementioned co-pending '608 application, the entirety of which isincorporated herein by reference.)

Example 12

Atomic Force Microscopy (AFM) Characterization of Graphene Flakes. Forgraphene flake characterization, a sample of graphene/EC dispersion inethanol was deposited onto Si/SiO₂ for AFM characterization. Prior tosample deposition, Si/SiO₂ wafers were immersed in 2.5 mM 3-aminopropyltriethoxysilane (Aldrich, 99%) in 2-propanol (Macron Chemicals, 99.5%)for 30 minutes, after which they were rinsed with 2-propanol and blowndry under a stream of N₂. A diluted graphene dispersion was dropcastonto the wafers and left for 10 minutes, after which it was blown drywith N₂ and rinsed with 2-propanol. To remove ethyl cellulose andresidual 3-aminopropyl triethoxysilane, the samples were annealed at400° C. in a tube furnace for 30 minutes. AFM images were obtained usinga Bruker ICON PT AFM System in tapping mode with a Veeco Model RTESP(MPP-11100-10) cantilever. The images were collected with 2 μm×2 μmscans, and particle characteristics were determined using NanoscopeAnalysis software. Flake thickness was determined from line scans, andflake area was measured automatically using the software. Flakethickness was measured for 355 flakes, and flake area was measured for216 flakes. (See FIGS. 9A-C.)

Example 13

Thermal Gravimetric Analysis (TGA) of Graphene/EC Powder. Powder samplesof pure ethyl cellulose and graphene/EC powder were analyzed using aMettler Toledo TGA/SDTA851 system at a heating rate of 5° C./min in air.(See FIGS. 10A-B.)

Example 14

Si/SiO₂ Surface Modification. Surface modification of Si/SiO₂ waferswith hexamethyldisilazane (HMDS, Aldrich, >99%) employed a vaportreatment technique. Si/SiO₂ wafers were cleaned by bath sonication inethanol for 20 minutes followed by 5 minutes O₂ plasma treatment(Harrick Plasma, Plasma Cleaner PDC-001). The wafers were then suspendedover a dish of HMDS in a contained vessel for 30 minutes, while the HMDSvapor coated the surface. The wafers were then rinsed with 2-propanoland dried under a stream of N₂. The resulting water contact anglewas˜66°.

Example 15

Ink Preparation and Printing. To prepare the ink for inkjet printing,graphene/EC powder was dispersed in an 85:15 cyclohexanone/terpineolmixture at a concentration of 2.4 wt % by bath sonication. The resultingink was passed through a 0.45 μm filter (Pall Acrodisc® CR 25 mm syringefilter, 0.45 μm PTFE Membrane) to remove any dust or contaminants thatcould destabilize printing. The ink was printed using a Fujifilm DimatixMaterials Printer (DMP-2800) equipped with a 10 pL drop cartridge(DMC-11610). The images of drop formation were captured using thebuilt-in camera of the printer (FIG. 8F). The shear viscosity of the inkwas measured using a Physica MCR 300 rheometer equipped with a 50 mmcone and plate geometry at shear rates of 10-1000 s⁻¹. The temperaturewas controlled by a Peltier plate for viscosity measurements at 25, 30,35 and 40° C. to evaluate the optimal printing temperature. The printingwas carried out at 30° C., for which the viscosity was 10-12 mPa·s atshear rates of 100-1000 s⁻¹ (FIG. 11). The surface tension was estimatedto be˜33 mN/m by the drop weight method. Calibration solvents included2-propanol, ethanol, deionized water and ethylene glycol.

Example 16

Scanning Electron Microscopy Characterization of Printed Features.Scanning electron micrographs of printed features following 250° C. and450° C. annealing were obtained on a Hitachi SU8030 Field Emission SEM.(See FIGS. 14A-D.)

Example 17

Annealing Study of Graphene Films. An ink containing graphene/ethylcellulose in ethanol/terpineol was prepared for blade-coating films.Graphene/ethyl cellulose powder (˜100 mg) was dispersed in 2 mL of 4:1ethanol/terpineol v/v by bath sonication. This ink was blade-coated ontoglass slides (VWR Micro Slides) into a 15×15 mm2 film defined by a maskof scotch tape. The sample was then annealed in a tube furnace (ThermoScientific, Lindberg Blue M). The sheet resistance of the resulting filmwas measured by a 4-point probe technique, employing the appropriategeometric correction factors, while the film thickness was measured byprofilometry (Dektak 150 Stylus Surface Profiler). These results wereused to calculate the resistivity plotted in FIG. 13A-B.

Example 18

Electrical Characterization of Printed Features. For electricalcharacterization, the printed graphene lines were annealed at 250° C.for 30 minutes. The line resistance was measured with Au probes. It wasverified that the line resistivity did not vary with measured linelength, indicating that these probes introduced a negligible contactresistance. The length of the lines was measured using opticalmicroscopy, such that the distance between the probes was used for theline length and not the total length of the printed line. The lineresistance was measured for six lines for each data point to provideerror bars. The line thickness and width were measured by AFM and usedto calculate resistivity of the printed features. For the line thicknessdata (FIG. 13D), the average thickness over the center 50% of the linewas taken as the line thickness.

Example 19

Flexibility Assessment. For printing on flexible substrates, polyimide(DuPont Kapton®, 125 μm) was cleaned prior to use by bath sonication inethanol for 20 minutes. Graphene lines were printed on the polyimidewith six printing passes using the same printing parameters as before.For electrical tests over many bending cycles (FIG. 15A), 30 mm lineswere used to enable handling while also ensuring that a largerproportion of the line was subject to mechanical stress. For theelectrical measurements in a flexed state (FIGS. 15B-D), 60 mm lineswere used to enable the experimental setup. The error bars were obtainedby measuring 8 lines for FIGS. 15A-B and 12 lines for FIG. 15C.

Example 20

High Shear Mixing for the Solution-Phase Exfoliation of Graphene. Asdiscussed above, the production of graphene for printed electronicsrequires large volumes of material to expand the scope of potentialapplications. Conventional methods employed in academic laboratories,particularly ultrasonication, have limited scalability due to the highenergy intensity required and the small process volumes. High shearmixing offers an attractive alternative with straightforward scaling tolarge volumes (˜m³) and energy-efficient exfoliation. The use of shearmixing in the production of a graphene/ethyl cellulose (EC) composite,for inkjet printing, is evaluated in FIG. 16. Higher grapheneconcentrations were achieved for a larger volume of dispersion usingshear mixing instead of ultrasonication, while the as-produced grapheneexhibited similar flake thickness and area. In conjunction herewith,shear mixing can be employed using apparatus, conditions and techniquesof the sort well-known to those skilled in the art and made aware ofthis invention.

For instance, shear mixing was performed using a Silverson L5M-ALaboratory Mixer with a 32 mm mixing head and square hole high shearscreen. 90.0 g natural graphite flake (Asbury Graphite Mills, 3061Grade) was dispersed in a solution of 18 g ethyl cellulose (EC) in 900mL ethanol (EC: Aldrich, viscosity 4 cP, 5% in toluene/ethanol 80:20,48% ethoxy; ethanol: Koptec, 200 proof). The dispersion was shear mixedfor 120 minutes at 10,230 rpm to produce graphene, with samplescollected at intervals and centrifuged for analysis. Such a procedurecan improve production rates by˜10×.

Example 21

Photonic Annealing of Graphene Patterns. Thermal annealing of agraphene/EC material can reduce applicability with respect to someplastic substrates with low glass transition temperatures. Photonicannealing, on the other hand, presents an alternative annealing strategycompatible with a broader range of substrates. By applying a rapid,intense light pulse, the graphene/EC material is selectively heated dueto its strong optical absorption while the transient nature of the pulselimits the heating of the substrate. To optimize the effectiveness ofphotonic annealing, graphene/EC films were printed with a high graphenecontent (e.g., ˜65% wt.). The sheet resistance of the films followingthermal and photonic annealing was measured for a range of annealingconditions. As shown in FIG. 17, photonic annealing yields a sheetresistance approximately 4× greater than that resulting from optimizedthermal annealing, while maintaining compatibility with plasticsubstrates limited to a maximum temperature of approximately 150° C.Such results show that process limitations relating to thermal annealingof a graphene/EC material can be mitigated. In conjunction herewith,photonic annealing can be employed using apparatus, conditions andtechniques of the sort well-known to those skilled in the art and madeaware of the present invention.

To illustrate photonic annealing, graphene/EC films were inkjet printedfrom an ink containing˜1.7 mg/mL graphene and˜0.85 mg/mL EC dispersed inan 85:15 mixture of cyclohexanone and terpineol. Printed films onpolyethylene terephthalate (PET, DuPont Teijin Films Melinex® ST579/200)were post-processed with photonic annealation using a Xenon Sinteron2000 pulsed light source, with a 1 ms light pulse at 2.4-3.6 kVoperation. Additional films on PET were first thermally annealed at 100°C. in air prior to the same photonic annealing treatment.

As demonstrated, above, the present invention provides a method forenhanced concentration of graphene and related nanomaterials to provide,in particular, graphene concentrations heretofore unrealized in the art.Such techniques are rapid and scalable, making more readily availablethe various mechanical, chemical and electronic attributes of suchmaterials over a wide range of end-use applications.

While the principles of this invention have been described inconjunction with certain embodiments, it should be understood clearlythat these descriptions are provided only by way of example and are notintended to limit, in any way, the scope of this invention. Forinstance, the present invention can be applied more specifically to thepreparation of concentrated carbon nanotube compositions and relatedcomposite materials, using methods of the sort described herein, or in amanner as described in conjunction with use of carbon nanotubes in theaforementioned and incorporated '608 reference. Likewise, the presentinvention can be used in conjunction with various flexible, bendablesubstrates. While polyimides and polyethylene terephthalates have beendescribed, substrates and corresponding composites can comprise and beprepared using various other flexible, bendable substrate materials, aswould be understood by those skilled in the art made aware of thisinvention.

We claim:
 1. A composite comprising a composition comprising anannealation product of a cellulose-stabilized graphene coupled to asubstrate.
 2. The composite of claim 1 wherein said substrate comprisesa flexible polymer.
 3. The composite of claim 2 wherein said substratecomprises a polymer selected from a polyimide and a polyethyleneterephthalate.